The crew launched promptly at 9.17pm in cloudy but good visibility conditions and swiftly made their way to Ardglass.
Arriving on scene around half an hour later, the crew began searching the area for the upturned boat — but soon discovered that the sighting was in fact of a dead whale that was floating on the surface of the Irish Sea.
Commenting on the callout, deputy launching authority Graham Edgar said: “This was an unfortunate conclusion of the search. However we are glad that no lives were in danger.
“Belfast Coastguard will report the whale as a hazard to navigation to ensure all other vessels in the area will be aware of the remains.”